A Veterinarian Expert answers some Questions on Science Diet y/d

Science Diet has yet to respond to my two requests for more information on their new prescription diet for hyperthyroid cats; y/d. But veterinarian expert Dr. Mark E. Peterson emailed me providing some professional insight on the new diet.

Dr. Mark E. Peterson specializes in pet endocrine problems such as diabetes, hyperthyroidism, hypothyroidism, Cushing’s and Addison’s disease. He read my post on Science Diet being slow to respond to questions about their new y/d diet, and was kind enough to share a post where he explains the problem of hyperthyroid disease in cats and his thoughts on the Science Diet y/d prescription food. Below are some highlights of his post…

“So, what’s the mechanism of action of this diet? How can something as simple as just lowering the iodine level in the food “cure” hyperthyroidism? Well, iodine metabolism is actually not that simple, and as we will discuss later, it is very clear that this diet does not represent a cure — it will only work to control the disease as long as the cat only eats this diet.

According to data provided by Hill’s Pet Nutrition, about 75% of hyperthyroid cat exclusively eating y/d will have normal serum total T4 concentrations by 4 weeks on the diet. By 8 weeks, 90% of cats have a serum T4 level; by 12 weeks, almost all cats should have normal values (1). This therapy appears to be more effective in cats with only moderate elevations of T4 than cats with severe hyperthyroidism.

Based on the data so far, however, the serum T4 concentrations in many cats fed this diet remain in the high-normal range, rather than the lower half of the T4 reference range. As you can see in Figure 3, the mean T4 values after feeding the diet fell to 40-45 nmol/l (3.1-3.5 μg/dl), in the upper half of the reference range limits (8). Based on our studies, as well of the studies of other investigators (9), most older, clinically normal cats have serum T4 values that run lower than that, typically in the lower end of the reference range (e.g., 10-30 nmol/L or 1.0-2.5 μg/dl). Therefore, in my opinion at least, that should be considered the ideal target range for success in treating cats with hyperthyroidism — no matter what therapy is used.

Overall, this data does indicate that feeding y/d, a diet severely restricted to overtly deficient in iodine, will result in normalization of T4 levels in most hyperthyroid cats. How well-controlled the hyperthyroid state will be maintained in cats fed y/d remains to be determined. We need additional studies to answer that question, as well as the long term safety aspect of feeding this iodine deficient diet.

Unfortunately for the practicing veterinarian, pet food diets are not FDA-regulated, so the company has not done the Phase II or III drug trials normally required to determine a drug’s efficacy or safety. It would have nice if more research had been done to answer these questions before its release onto the market.

Based on the information we have thus far, however, this diet certainly does provide us with another option for medical management of this very common disease. But with y/d and other prescription diets, shouldn’t we be also be looking at the long-term heath benefits and disadvantages for the whole cat?”

I asked Science Diet to explain how their Nature’s Best ‘beat’ these other foods. What were the circumstances of the taste test. I’ve gotten no response to this question either. I guess Science Diet is not speaking to me. That’s a shame; they don’t have to like me (or what I do), but pet owners do deserve to have their questions answered.